Patent Literature 1: JP 2004-308752 A (US 2006/0207373 A1)
There is conventionally known in vehicle controls a by-wire system to permit a by-wire control circuit to perform electrical control on actuators that change vehicle states according to instructions from drivers of vehicles. For example, Patent Literature 1 discloses a shift-by-wire system to switch the shift range according to driver's instructions. This system controls the drive of an actuator with a brushless motor and rotates a detent plate in the shift-range switching apparatus, switching the shift range.
The shift-by-wire system in Patent Literature 1 switches the shift range to a target range by rotating the motor to a target rotational position corresponding to the target range based on a counted value of pulse signals from an encoder that outputs the pulse signals every predetermined rotational angle in synchronization with the rotation of the motor. In addition, the shift-by-wire system in Patent Literature 1 performs a reference position learning control that learns a reference position of the motor by rotating the motor until the detent plate stops at a limit position in a working span, before starting the control of the shift range (i.e., before starting a usual drive control of the motor). Learning the reference position permits the limit position to accord with the reference position of the motor, subsequently enabling a usual drive control that rotates the motor to a target rotational position.
There is a system that controls electric currents flowing through each phase of the motor based on a duty ratio depending on voltages when a reference position learning control is performed. In such a system, a current value of electric currents flowing through each phase of the windings of the motor changes with the change of the winding resistance due to temperature variations, time-based variations, etc. even if the voltage is maintained unchanged; the changed current value poses the change in the torque of the motor. This may deteriorate the accuracy in learning the reference position. Such an issue is not considered by the system in Patent Literature 1. The system in Patent Literature 1 may therefore deteriorate the accuracy in learning a reference position because of such condition variations as temperature variations or time-based variations.
In order to improve the accuracy in learning of a reference position, the electric currents flowing through the windings may be limited such that the sum of the electric currents flowing through the respective winding phases of the motor is kept within a predetermined span; this suppresses the variation in the current value of each phase due to the temperature variations or time-based variations. A physical (i.e., hardware) circuit may be employed to form a device detecting the sum of the electric currents that flow through each winding phase of the motor, or a device restricting the electric currents that flow through the windings. Such a physical circuit may however break down, disabling accurate learning of a reference position.